Purification of propylene oxide by cosolvent extractive distillation

ABSTRACT

An extractive distillation agent consisting essentially of a mixture of triethylene glycol with a lower boiling cosolvent selected from the group consisting of 2-methyl-2,4-pentanediol, tertiary butyl alcohol and ethylene glycol is fed to an extractive distillation column used for the distillation of propylene oxide contaminated with water, acetone and methanol to obtain an overhead distillate fraction consisting of essentially anhydrous propylene oxide contaminated with reduced quantities of acetone and methanol, and a heavier bottoms distillation fraction containing substantially all of the extractive distillation agent, water and acetone and some of the methanol introduced into the distillation column.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

This invention relates to the purification of propylene oxide. More particularly, this invention relates to a distillation process for removing contaminating quantities of impurities including oxygen-containing impurities such as methanol, acetone and water from an impure propylene oxide feedstock. Still more particularly, this invention relates to a method wherein an impure propylene oxide feedstock contaminated with from about 50 to about 4,000 ppm of methanol, from about 0.01 to about 2 wt. % of water and from about 0.01 to about 2 wt. % of acetone is purified in an extractive distillation column using as the extractive distillation agent, a mixture of triethylene glycol with a lower boiling solvent selected from the group consisting of 2-methyl-2,4-pentanediol, tertiary butyl alcohol and ethylene glycol.

2. Prior Art

In copending Marquis et al. U.S. patent application Ser. No. 07/770,493, filed Oct. 3, 1991 and entitled "Purification of Propylene Oxide by Extractive Distillation using Triethylene Glycol" (D#81,112), there is disclosed, as indicated by the title, a method for the purification of propylene oxide wherein triethylene glycol is used as the extractive distillation agent.

It is known to react a hydroperoxide feedstock such as tertiary butyl hydroperoxide with propylene in the presence of an epoxidation catalyst in order to provide a reaction product comprising propylene oxide, an alcohol corresponding to the hydroperoxide feedstock, a solvent, and impurities (see, for example, Kollar U.S. Pat. No. 3,350,422, Kollar U.S. Pat. No. 3,351,635 and Sorgenti U.S. Pat. No. 3,666,777.

It is also known to separate the reaction product by distillation in order to obtain a plurality of fractions including, for example, a propylene recycle fraction, a propylene oxide product fraction, an alcohol fraction, etc.

It is also known that methanol, acetone and water are common contaminants for propylene oxide which are removed only with difficulty.

For example, Mitchell et al. U.S. Pat. No. 2,550,847 is directed to a process for separating purified propylene oxide from a crude propylene oxide product contaminated with acetaldehyde, methyl formate, methanol, etc., by treating the crude mixture with an aqueous basic substance followed by recovery of the purified propylene oxide by any suitable means such as by decantation. Mitchell et al. reported a recovery of a product containing 78 to 82 wt. % of propylene oxide which, they stated, could be increased in purity to about 95 to 99% by fractional distillation.

Robeson et al. U.S. Pat. No. 2,622,060 discloses a process for the purification of propylene oxide contaminated with impurities, including methanol, by subjecting the impure propylene oxide to distillation in the presence of an extractive distillation agent comprising an aqueous solution of an alkali. The inventors report in Example 1 of their patent a method wherein 500 parts by weight of a crude propylene oxide fraction was extractively distilled in accordance with their invention to obtain 325 parts by weight of a product containing about 99.6 wt. % of propylene oxide. Thus, a significant loss of propylene oxide occurred during the process.

In a process unrelated to the purification of propylene oxide, Goddin et al. in U.S. Pat. No. 2,751,337 disclose a process for separating acetone from a mixture of acetone with methanol and methyl acetate utilizing water as an extractive distillation agent.

Hamlin et al. in U.S. Pat. No. 3,409,513 disclose the hydro-extractive distillation of mixtures comprising acetone, lower aliphatic alcohols and esters of lower aliphatic alcohols with carboxylic acids. It is pointed out by the patentees that acetone, methyl acetate and methanol form an azeotrope boiling at 55.5°-56.5° C. Hamlin et al. propose to recover partially purified acetone from such a ternary azeotrope by liquid-liquid extraction with water followed by hydro-extractive distillation of the aqueous phase in order to obtain a partially purified acetone fraction.

Washall U.S. Pat. No. 3,578,568 discloses a process for removing oxygen-containing impurities such as acetone, acetaldehyde and methanol from impure propylene oxide using a glycol such as ethylene glycol or propylene glycol as an extractive distillation agent.

Hoorl and Newman U.S. Pat. No. 3,632,482 is directed to a propylene oxide recovery process by extractive distillation using an alcohol-ketone-hydrocarbon solvent. The invention relates to a method for the purification of crude propylene oxide contained in a mixture produced by the epoxidation of propylene with an organic hydroperoxide and calls for extractive distillation of the crude propylene oxide in a plurality of successive extractive distillation zones with the aid of a solvent mixture consisting essentially of hydrocarbons, alcohols, and/or ketones corresponding to the organic hydroperoxide employed in producing the propylene oxide. In the preferred embodiment of their invention, the extractive distillation agent is a recycle fraction from a three column distillation sequence wherein the bottoms from the third distillation column are flashed to obtain an overhead composed of hydrocarbons, alcohols and/or ketones which is recycled as an extractive distillation agent to the three distillation columns involved in the propylene oxide purification sequence.

Burns et al. U.S. Pat. No. 3,715,284 discloses a process for the purification of impure propylene oxide using acetone or a mixture of acetone with methanol which is introduced into a distillation column either below or together with the impure propylene oxide.

Schmidt U.S. Pat. No. 3,881,996 is directed to a distillation sequence employing at least three and optionally four columns for the purification of crude propylene oxide, one of the columns optionally being an extractive distillation column wherein a hydrocarbon such as octane is used as the extractive distillation agent.

Schmidt U.S. Pat. No. 4,140,588 discloses a process for the purification of propylene oxide contaminated with methanol and acetone using water as an extractive distillation agent, the water being introduced into the distillation column above the point of introduction of the crude propylene oxide feed.

Schmidt states at column 2, lines 50-55 that: "Propylene oxide, however, has a substantial solubility in water and is readily hydrolyzed to propylene glycol (PG) in the presence of large amounts of water"--i.e., in the reboiler section of the tower.

U.S. Pat. No. 4,971,661 discloses the use of an aqueous acetone extraction to remove methanol from propylene oxide.

U.S. Pat. No. 3,578,568 discloses the use of glycols or glycol ethers in an extractive distillation to remove oxygen containing impurities such as acetone, acetaldehyde, and methanol. It is claimed that the concentration of the solvent in the vapor space in the extractive distillation zone of the distillation tower is preferably between 15 and 50 mole percent of the total vapor.

Compared to U.S. Pat. No. 3,578,568, this invention uses considerably lower concentrations of solvent in the extractive distillation zone to remove water and oxygen-containing impurities such as acetone. Since the concentration of the triethylene glycol is lower, the size and heat requirements of the associated triethylene glycol regenerator are reduced.

U.S. Pat. No. 4,140,588 discloses the use of water in an extractive distillation to remove methanol and acetone.

U.S. Pat. No. 3,607,669 discloses the use of a C₈ to C₁₂ hydrocarbon to separate propylene oxide from water.

Shih et al. U.S. Pat. No. 5,000,825 discloses the purification of monoepoxides such as propylene oxide that are contaminated with oxygenated impurities such as water, low molecular weight alcohols, low molecular weight ketones, low molecular weigh aldehydes and the like by the extractive distillation of the contaminated monoepoxide using a lower glycol containing 2 to 4 carbon atoms. Examples of lower glycols that are given in the patent include ethylene glycol, 1,2-propane diol, 1,3-propane diol, 1,4-butane diol, 1,2-butane diol, 1,3-butane diol and 2,3-butane diol. It is stated that higher diols or higher glycol ethers do not provide sufficient selectivity for the removal of such impurities and are not included as the extractive distillation solvents suitable for use in the invention.

SUMMARY OF THE INVENTION

In accordance with the present invention, an impure propylene oxide feedstock contaminated with 0.01 to 2 wt. % of water, from about 50 to about 4,000 ppm of methanol and from about 0.01 to about 2 wt. % of acetone is charged to the lower half of an extractive distillation column containing at least about 10 theoretical plates and an extractive distillation agent consisting essentially of a mixture of triethylene glycol with a lower boiling solvent selected from the group consisting of 2-methyl-2,4-pentanediol, tertiary butyl alcohol and ethylene glycol and is charged to the tower at a point at least 4 stages above the impure propylene oxide feed point. Preferably, the extractive distillation tower will contain from about 30 to about 120 theoretical plates and the extractive distillation agent will be charged to the tower at a point of from 7 to 50 theoretical stages above the impure propylene oxide feed point. The extractive distillation agent is introduced into the extractive distillation column in the ratio of said feedstock to said extractive distillation agent of from about 1:1 to about 20:1, and more preferably 2:1 to 10:1, whereby a light distillate fraction is obtained consisting essentially of propylene oxide contaminated with significantly reduced amounts of water, methanol and acetone, such as about 5 to about 600 ppm of water, about 15 to 2,000 ppm of methanol and about 0.1 to about 100 ppm of acetone.

BACKGROUND OF THE PRESENT INVENTION

When propylene is reacted in liquid phase with an organic hydroperoxide such as tertiary butyl hydroperoxide in solution in a solvent such as tertiary butyl alcohol in the presence of a soluble epoxidation catalyst such as a molybdenum epoxidation catalyst, a reaction mixture is formed comprising propylene oxide, an alcohol corresponding to the organic hydroperoxide feedstock and impurities including water and other oxygenated impurities such as methyl formate, acetaldehyde, acetone and methanol.

Propylene oxide is a hygroscopic substance, so that water is removed only with difficulty. It is important to remove as much of the water as possible, however, because the water present in the propylene oxide will tend to react with the propylene oxide to form propylene glycol.

It is also important to reduce the level of other oxygenated contaminants such as methanol and acetone to the lowest reasonably attainable level.

In accordance with conventional practice, an epoxidation reaction product formed by the molybdenum-catalyzed reaction of propylene oxide with tertiary butyl hydroperoxide in solution in tertiary butyl alcohol is separated into the principle components by distillation so as to form distillation fractions including a propylene distillation fraction, a propylene oxide distillation fraction, a tertiary butyl alcohol distillation fraction and a heavy distillation fraction containing the molybdenum catalyst and other products and by-products of the epoxidation reaction. However, the distillation fractions that are thus-obtained are characterized by the inclusion of impurities and, normally, must be further treated if commercially acceptable products are to be obtained. This is especially true for a propylene oxide distillation fraction contaminated with water and oxygenated contaminants including methanol and acetone.

It has been surprisingly discovered in accordance with the present invention that substantially all of the water initially present in a contaminated propylene oxide feedstock can be removed therefrom when the propylene oxide feedstock is extractively distilled in the presence of an extractive distillation agent consisting essentially of a mixture of triethylene glycol with a lower boiling solvent selected from the group consisting of 2-methyl-2,4-pentanediol, tertiary butyl alcohol and ethylene glycol. Also, surprising is our discovery that substantially all of the acetone and most of the methanol present in the contaminated feedstock can be removed from the propylene oxide when using an extractive distillation agent consisting essentially of a mixture of triethylene glycol with a lower boiling solvent selected from the group consisting of 2-methyl-2,4-pentanediol, tertiary butyl alcohol and ethylene glycol.

It has also been discovered that when an extractive distillation column is operated at ambient pressure in the purification of propylene oxide using triethylene glycol as the extractive distillation agent, the temperature required at the bottom of the tower to effect the extractive distillation can be at, or near, or even above the thermal degradation temperature of the triethylene glycol.

In accordance with the present invention, the triethylene glycol thermal degradation and other problems are resolved through the use of an extractive distillation agent consisting essentially of a mixture of triethylene glycol with a lower boiling solvent selected from the group consisting of 2-methyl-2,4-pentanediol, tertiary butyl alcohol and ethylene glycol.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a schematic flow sheet with conventional parts omitted showing the general recovery sequence that is used in accordance with the present invention in purifying propylene oxide.

In the drawing, for convenience, the present invention is illustrated in connection with a process wherein the propylene oxide is prepared by the epoxidation of propylene with tertiary butyl hydroperoxide in solution in tertiary butyl alcohol to provide a reaction product comprising propylene oxide and additional tertiary butyl alcohol.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawing, there is shown a schematic flow sheet illustrating a preferred method of practicing the process of the present invention. In the drawing, conventional parts such as valves, pumps, temperature sensors, pressure sensors, heaters, coolers, flow control regulation apparatus, etc., have been omitted.

In accordance with a preferred embodiment of the present invention, propylene oxide is separated in a preliminary distillation zone (not shown) from other components of an epoxidation reaction mixture in order to provide an impure propylene oxide fraction contaminated with oxygen-containing impurities such as acetone, methanol, water, etc.

The impure propylene oxide feedstock that is thus obtained in the preliminary distillation zone is then purified in a propylene oxide purification distillation zone, which in accordance with the preferred embodiment of the present invention, comprises two distillation columns, each of which is equipped with an appropriate reflux condensing means and an appropriate reboiler heating means.

In accordance with the present invention, an impure propylene oxide fraction contaminated with from about 50 to about 4,000 ppm of methanol, from about 0.01 to about 2 wt. % of acetone and about 0.01 to about 2 wt. % of water and other oxygen-containing impurities is charged by way of a line 120 leading to a distillation column 100 which, in accordance with the present invention, will preferably be a column containing at least about 10 theoretical plates and more preferably, from about 30 to about 100 theoretical plates. The column 100 is suitably operated under distillation conditions including a pressure of about 10 to 40 psia, a reflux ratio of from about 2:1 to about 10:1, a reboiler temperature within the range of about 100° to about 250° C. (e.g., 210° C.) and a top temperature of about 20° to about 80° C. (e.g., about 20° C.).

The impure propylene oxide is preferably charged to the distillation column 100 in the lower half thereof. An extractive distillation agent consisting essentially of a mixture of triethylene glycol with a lower boiling solvent selected from the group consisting of 2-methyl-2,4-pentanediol, tertiary butyl alcohol and ethylene glycol and is charged to the upper half of the distillation column 100 by an extractive distillation charge line 106.

Essentially anhydrous purified propylene oxide containing about 100 ppm or less of water is removed from the column 100 as a light distillation fraction 112, the purified propylene oxide in the line 112 containing significantly reduced-amounts of methanol and acetone, such as about 15 to 900 ppm of methanol and about 0.1 to 100 ppm of acetone. A heavier fraction 110 is withdrawn from the distillation column 100 which contains substantially all of the extractive distillation agent charged by the line 106 and also substantially all of the water, acetone and other oxygen-containing impurities introduced into the column 100 with the impure propylene oxide 120.

The heavier distillation fraction 110 from the column 100 comprising water, methanol, acetone, tertiary butyl alcohol and other impurities and extractive distillation agent is charged to a second distillation column 200 wherein light impurities such as methanol, acetone, water, etc., are separated overhead as a distillation fraction 204 that is discharged from the system for any suitable use, such as for use as a steam boiler feedstock or for recovery.

A heavier distillation fraction 106 is discharged from the distillation column 200 comprising essentially of a mixture of triethylene glycol with a lower boiling solvent which is recycled to distillation column 100 by line 106.

In accordance with the present invention, fresh extractive distillation agent is introduced into the system by a branch line 230 leading to the charge line 106 for the first distillation column 100 so that any water introduced into the system with the fresh extractive distillation agent will be charged to the second column 200 by the line 100 and withdrawn from the column 200 through the line 204.

The Extractive Distillation Agent

Triethylene glycol is a compound having the formula:

    HO--CH.sub.2 CH.sub.2 --O--CH.sub.2 CH.sub.2 --O--CH.sub.2 CH.sub.2 --OH

Triethylene glycol is a colorless, hygroscopic, practically odorless liquid having a boiling point of 287.4° C., a vapor pressure of less than 0.01 mm (20° C.) and a specific gravity of 1.1254 (20/20° C.). Technical grades of triethylene glycol will contain an appreciable amount of water (e.g., about 0.01 to about 0.1 wt. %). Therefore, if fresh technical grade triethylene glycol were introduced directly into the column 100, a substantial amount of undesired contaminating water would also be introduced.

2-Methyl-2,4-pentanediol is a compound having the formula: ##STR1##

2-Methyl-2,4-pentanediol is a colorless liquid that is completely miscible with water and many organic solvents. It has a boiling point of 197° C., a specific gravity of 0.9235 (20/20° C.) and a flash point of 93.9° C. Technical grades of 2-methyl-2,4-pentanediol will contain water and if introduced directly into column 100 would add to the level of contaminants introduced into the column.

Tertiary butyl alcohol (2-methyl-2-propanol) is a compound having the formula:

    (CH.sub.3).sub.3 COH

Tertiary butyl alcohol is a colorless liquid having a camphor odor, a freezing point of 25.5° C. a boiling point of 2.9° C., a specific gravity at 26° C. of 0.779 and is miscible with water and ethanol.

Ethylene glycol (1,2-ethanediol) is a compound having the formula:

    HO--CH.sub.2 CH.sub.2 --OH

Ethylene glycol is a colorless, hygroscopic, water-soluble liquid having a specific gravity of 1.1155 (20° C.), a boiling point of 197.2° C. a freezing point of -13.5° C. and a flash point of 116° C.

The 2-methyl-2,4-pentanediol, tertiary butyl alcohol and ethylene glycol (the cosolvent) are suitably used in the weight ratio to primary solvent of about 0.1:1 to about 1:1, and more preferably in the weight ratio of about 0.5:1 to about 1:1.

The magnitude by which the bottoms temperature is depressed is proportional to the concentration of the co-solvent in the triethylene glycol.

EXAMPLES

The invention will be further illustrated by the following specific examples which are given by way of illustration and not as limitations on the scope of this invention. Where parts are mentioned, they are parts by weight.

Simulations were performed using Simulation Sciences "Process" simulation program. The input consisted of thermodynamic and physical property data generated for Texaco Chemical's PO/MTBE project. A sixty (60) theoretical column was used with the solvent feed point located at tray ten (10) and the impure PO feed point located at tray forty (40). Specifications for the simulation were a reflux ratio of 2.5, a crude PO feed rate of 50,000 pounds per hour, an extractive solvent feed rate of 10,000 pounds per hour, a condenser pressure of 20 psia, and a PO loss out the bottom of the column of 1.5 pounds per hour. Reboiler and condenser duties were varied to meet these specifications.

When TEG (tri-ethylene glycol) was used as the initial solvent for the removal of water, methanol, and acetone from PO, the resulting bottoms temperature is predicted to be 376.5° F. at a reboiler pressure of 36 psia.

Approximately 99% of the methanol, 100% of the acetone, and 100% of the water in the crude PO feed to the column is removed in the bottoms stream.

Due to a potential high bottoms temperature in the column, a cosolvent, 2-methyl-2,4-pentanediol was added to the primary solvent feed at tray ten (10). The results indicated that the bottoms temperature of the column was reduced while the concentration of the water and other impurities in the overhead product was essentially unchanged. For example, when a 1:1 mixture of 2-methyl-2,4-pentanediol and triethylene glycol is used as the extractive distillation solvent, the reboiler temperature decreases to about 360° F. while the efficiency of the removal of water, acetone, and methanol does not decrease.

When the triethylene glycol/2-methyl-2,4-pentanediol cosolvent extractive distillation is simulated, approximately 98% of the methanol, 100% of the acetone, and 100% of the water in the crude PO feed to the column is removed in the bottoms stream.

Using distillation simulation conditions as in the previous example, an extractive distillation solvent consisting of a 1:1 mixture of triethylene glycol and tertiary butyl alcohol gives a bottoms temperature of 238° F. Approximately 88.5% of the methanol, 100% of the acetone, and 98.6% of the water in the crude PO feed to column is removed in the bottoms stream.

Using distillation simulation conditions as in the previous example, an extractive distillation solvent consisting of a 1:1 mixture of triethylene glycol and ethylene glycol gives a bottoms temperature of 338° F. Approximately 99.2% of the methanol, 100% of the acetone, and 99.8% of the water in the crude PO feed to column is removed in the bottoms stream.

Water and oxygen-containing impurities such as acetone and methanol are difficult to remove from propylene oxide by standard distillation. The use of extractive distillation columns with a mixture of triethylene glycol with a lower boiling solvent selected from the group consisting of 2-methyl-2,4-pentanediol, tertiary butyl alcohol and ethylene glycol and improves the separation of these impurities from propylene oxide. 

Having thus described our invention, what is claimed is:
 1. An extractive distillation process for the distillation of an impure propylene oxide feedstock in a distillation column to remove oxygenated contaminants, including water, methanol, and acetone from the impure propylene oxide feedstock which comprises the steps of:introducing said impure propylene oxide feedstock into the lower half of a distillation column, said impure propylene oxide feedstock comprising propylene oxide contaminated with from about 0.01 to about 2 wt. % of water, from about 50 to about 4000 ppm of methanol and from about 0.01 to about 2 wt. % of acetone, introducing an extractive distillation agent consisting essentially of a mixture of triethylene glycol with a lower boiling solvent selected from the group consisting of 2-methyl-2,4-pentanediol, tertiary butyl alcohol and ethylene glycol above the said impure propylene oxide feedstock feed point, said extractive distillation agent being introduced into said distillation column in the ratio of said feedstock to said extractive distillation agent of from about 1:1 to about 20: 1, withdrawing an overhead distillate fraction from said distillation column consisting essentially of essentially anhydrous propylene oxide contaminated with reduced quantities of acetone and methanol, withdrawing a bottoms distillation fraction from said distillation column containing substantially all of the extractive distillation agent, water and acetone and a portion of the methanol introduced into said distillation column.
 2. An extractive distillation process for the distillation of an impure propylene oxide feedstock in a distillation column to remove contaminants, including water, methanol and acetone, which comprises the steps of:charging said impure propylene oxide feedstock to the lower half of a distillation column containing at least 25 theoretical plates, said impure propylene oxide feedstock comprising propylene oxide contaminated with from about 0.01 to 2 wt. % of water, from about 50 to about 4000 ppm of methanol and from about 0.01 to about 2 wt. % of acetone, introducing an extractive distillation agent consisting essentially of a mixture of triethylene glycol with a lower boiling solvent selected from the group consisting of 2-methyl-2,4-pentanediol, tertiary butyl alcohol and ethylene glycol at a point at least 4 theoretical plates above the said impure propylene oxide feedstock feed point, said extractive distillation agent being introduced into said distillation column in the ratio of said feedstock to said extractive distillation agent of from about 1:1 to about 20:1, withdrawing an overhead distillate fraction from said distillation column consisting essentially of essentially anhydrous propylene oxide contaminated with reduced quantities of acetone and methanol, and withdrawing a bottoms distillation fraction from said distillation column containing substantially all of the extractive distillation agent, water and acetone and a portion of the methanol introduced into said distillation column.
 3. An extractive distillation process for the distillation of an impure propylene oxide feedstock in a distillation column, which comprises the steps of:introducing an impure propylene oxide feedstock into the lower half of a distillation column containing at least 25 theoretical plates, said impure propylene oxide feedstock comprising propylene oxide contaminated with from about 0.01 to about 2 wt. % of water, from about 50 to about 4000 ppm of methanol and from about 0.01 to about 2 wt. % of acetone, introducing an extractive distillation agent consisting essentially of a mixture of triethylene glycol with a lower boiling cosolvent selected from the group consisting of 2-methyl-2,4-pentanediol, tertiary butyl alcohol and ethylene glycol at a point at least 4 theoretical plates above the said impure propylene oxide feed point, said extractive distillation agent being introduced into said distillation column in the ratio of said feedstock to said extractive distillation agent of from about 1:1 to about 20:1, fractionating said impure propylene oxide feedstock in said distillation column under distillation conditions including a pressure of about 10 to about 40 psia, a reflux ratio of from about 1:1 to about 5:1, and a reboiler temperature within the range of about 100° to about 250° C. and a top temperature of about 20° to about 80° C., withdrawing an overhead purified propylene oxide distillate fraction from said distillation column consisting essentially of anhydrous propylene oxide, said purified propylene oxide distillate fraction being contaminated with significantly reduced amounts of methanol and acetone, and withdrawing a heavier distillation fraction from said distillation column containing not more than about 1 wt. % of the propylene oxide charged to said distillation column and containing substantially all of the extractive distillation agent, water, methanol and acetone introduced into said distillation column.
 4. A method as in claim 3 wherein the weight ratio of triethylene glycol to cosolvent is in the range of about 0.1:1 to about 1:1.
 5. A method as in claim 4 wherein the weight ratio of triethylene glycol to cosolvent is in the range of about 0.5:1 to about 1:1.
 6. A method as in claim 5 wherein the cosolvent is 2-methyl-2,4-pentanediol.
 7. An extractive distillation process for the distillation of an impure propylene oxide feedstock in a distillation column, which comprises the steps of:introducing an impure propylene oxide feedstock into the lower half of a distillation column containing at least 25 theoretical plates, said impure propylene oxide feedstock comprising propylene oxide contaminated with from about 0.01 to about 2 wt. % of water, from about 50 to 4000 ppm of methanol and from about 0.01 to about 2 wt. % of acetone, introducing an extractive distillation agent consisting essentially of a mixture of triethylene glycol with tertiary butyl alcohol at a point at least 4 theoretical plates above the said impure propylene oxide feed point, said extractive distillation agent being introduced into said distillation column in the ratio of said feedstock to said extractive distillation agent of from about 1:1 to about 20:1, the weight ratio of triethylene glycol to tertiary butyl alcohol being in the range of about 0.5: 1 to about 1:1 fractionating said impure propylene oxide feedstock in said distillation column under distillation conditions including a pressure of about 10 to about 40 psia, a reflux ratio of from about 1:1 to about 5:1, a reboiler temperature within the range of about 100° to about 250° C., and a top temperature of about 20° to about 80° C., withdrawing an overhead purified propylene oxide distillate fraction from said distillation column consisting essentially of anhydrous propylene oxide, said purified propylene oxide distillate fraction being contaminated with significantly reduced amounts of methanol and acetone, and withdrawing a heavier distillation fraction from said distillation column containing not more than about 1 wt. % of the propylene oxide charged to said distillation column and containing substantially all of the extractive distillation agent, water, methanol and acetone introduced into said distillation column.
 8. A method as in claim 5 wherein the cosolvent is ethylene glycol. 